1. Field of the Invention
The present invention relates to non-linear signal processing and, in particular, to the removal of undesired harmonics created by such processing.
2. Description of the Related Art
The present invention will be discussed with reference to video signal processing in TV receivers but is not limited to that field. It has wide applicability wherever signal samples are subjected to non-linear processing, as, again for example, in the medical instruments field.
In TV receivers, efforts to improve video images, have, in the recent past, concentrated to a considerable extent on improving the perceived image by decreasing or eliminating line flicker and field flicker. When the television picture is an interlaced picture these problems have been attacked in two basic ways. First, to eliminate line flicker, additional lines are interpolated between consecutive lines in each field. This converts the TV images from an interlaced format to a progressive scan. To remove the field or frame flicker, fields are interpolated between the consecutive fields by utilizing data from one or the other original raster, or both, combined non-linearly by, for example, a median filter. Non-linear filters are also used for filtering in the horizontal direction, e.g. for increasing the perceived image resolution. An example of use of a median filter for field interpolation is shown in EP-A-0,192,292 (FIG. 7).
Since use of the vertical median filters still causes distortion in the video picture, an analysis of the problem was undertaken with the following result. For a non-orthogonal picture pattern as illustrated in FIG. 1a, the original signal components in the vertical and horizontal direction are illustrated in FIG. 1b and FIG. 1c, respectively. Use of a median filter in the vertical direction will result in a signal pattern as illustrated in FIG. 1d. A similar artefact will be created in the horizontal direction, as illustrated in FIG. 1e. Uneven harmonics have been created, of which the third harmonic is by far the strongest.
The above-mentioned harmonics constitute a first order effect which, however, is not as disturbing as a second order effect resulting from the beat frequency or alias components between the above-mentioned odd harmonics and the frequency at which the signal is sampled in the horizontal direction, for example to permit digital signal processing. As shown in FIG. 1f, if a signal having a third harmonic component is sampled at three times the fundamental frequency, a DC level shift will occur, thus changing the average luminance value of the signal.
A realistic example of a situation in which this kind of beat frequency can occur is the following. A PAL video signal includes undesired remainders of the color subcarrier at 4.43 MHz which form a diagonal pattern. The third harmonic of this pattern will be at 13.3 MHz. If now this signal is sampled in the horizontal direction at 13.5 MHz, the resulting beat frequency will be 200 KHz. If the color carrier traces extend over the whole picture, a pattern having 10 periods per line results, which is very disturbing.
The problem arises because the sampling frequency is chosen so that the frequency spectrum of the video signal extends to 1/2 of the sampling frequency. This results in the greatest economy with respect to required memory and also in the lowest operating speed. Thus signal components of 1/3 of the sampling frequency may well lie within the spectrum of the video signal.
In general, a solution to this problem has been to increase the sampling frequency so that the spectrum of the video signal remains well below 1/3 of the sampling frequency. This is not an economical solution since, for example, the capacity of the field memories or frame memories has to be increased by 50% and circuits have to operate at the higher frequency.